The field of the invention is nuclear magnetic resonance (NMR) techniques for measuring the properties of materials and producing images.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characterisitic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented components in the perpendicular plane (x-y plane) cancel on another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the excitation signal B.sub.1 is terminated. When the excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of M.sub.1. The amplitude A of the emission signal (in simple systems) decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on the T.sub.2 of the particular nuclei which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportioned to te exponential rate at which the aligned precession of the nuclei dephase after the removal of the excitation signal B.sub.1. The measurement of T.sub.2 or the modulation of NMR signals by T.sub.2 effects provides valuable information about the substance under study.
Other factors contribute to the amplitude of the free induction decay (FID) signal which is defined by the T.sub.2 spin-spin relaxation process. One of these is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value M.sub.O along the axis of magnetic polarization (Z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances, and its independent measurement is the subject of many procedures.
The measurements described above are called "pulsed NMR measurements." They are divided into a period of excitation and a period of emission. As will be discussed in more detail below, this measurement cycle may be repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject.
Although NMR measurements are useful in many scientific and engineering fields, their primary use is in the field of medicine. NMR measurements provide a contrast mechanism which is quite different from X-rays, and this enables difference between soft tissues to be observed with NMR which are completely indiscernible with X-rays. In addition, physiological differences can be observed with NMR measurements, whereas X-rays are limited particularly to anatomical studies.
For most medical applications utilizing NMR, an imaging technique must be employed to obtain information at specific locations in the subject. The foremost NMR imaging technique is referred to as "zeugmatography" and was first proposed by P. C. Lauterbur in a publication "Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance", Nature, Vol. 242, Mar. 16, 1973, pp. 190-191. Zeugmatography employs one or more additional magnetic fields which have the same direction as the polarizing field B.sub.0, but which have a nonzero gradient. By varying the strength (G) of these gradients, the net strength of the polarizing field B.sub.0 =B.sub.z +G.sub.x X+G.sub.y Y+G.sub.z Z at any location can be varied. As a result, if the frequency response of the receiver coil and circuitry is narrowed to respond to a single frequency, .omega..sub.0, then gyromagnetic phenomena will be observed only at a location where the net polarizing field B.sub.0 is of the proper strength to satisfy the Larmor equation; .omega..sub.0 =.gamma.B.sub.0 : where .omega..sub.0 is the Larmor frequency at that location.
By "linking" the resulting NMR signal with the strengths of the gradients (G=G.sub.x, G.sub.y, G.sub.z) at the moment the signal is generated, the NMR signal is "tagged", or "sensitized", with position information. Such position sensitizing of the NMR signal enables an NMR image to be reconstructed from a series of measurements. Such NMR imaging methods have been classified as point methods, line methods, plane methods and three dimensional methods. These are discussed, for example, by P. Mansfield and P. G. Morris in their book NMR Imaging in Biomedicine published in 1982 by Academic Press, New York.
The NMR scanners which implement these techniques are constructed in a variety of sizes. Small, specially designed machines are employed to examine laboratory animals or to provide images of specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof.
The NMR behavior of the sodium-23 nucleus in vivo is a complex problem which has generated considerable medical interest. The sodium cation is one of the most abundant ions in the human body, second only to the hydrogen nucleus in local concentration. There are major differences between concentrations of sodium in the intracellular cytoplasm and in the cell nucleus. The sodium concentration gradient between intracellular and extracellular sodium is maintained by the sodium potassium pump. This close relationship between intracellular and extracellular sodium concentration with membrane permeability and the adenosine triphosphate powered Na-K pump makes sodium a sensitive indicator of cellular change and death.
Initially, NMR measurements of sodium were performed on continuous wave spectrometers using excised tissue samples. These studies compared the total sodium in the tissue as determined from the NMR measurement with the total sodium determined by ashing the tissue. These studies found that approximately 60% of the total sodium was not detected using the NMR measurement technique. Further study revealed that the sodium nucleus exhibited a two component T.sub.2 relaxation constant. This bi-exponential T.sub.2 relaxation has a short component of from 0.7 to 4.8 milliseconds and a long component of from 7.0 to 26 milliseconds. This two component T.sub.2 relaxation process is thought to originate from quadrupolar interactions of the sodium nucleus with the surrounding electric fields. The short component is estimated to be attributable to 62% to 68% of the total sodium present in the tissue. The discrepancy in prior NMR measurements of sodium is, therefore, due to the inability to produce an NMR signal quickly enough to measure the short component.